In heating appliances, such as toasters and toaster ovens, a food item is placed in a bread cavity of the appliance and toasted for a desired time, which is known as the heating cycle of the appliance. The duration of the heating cycle determines the extent to which the food item is cooked or toasted. For example, in a conventional toaster, the time required to toast successive food items to the same extent decreases for each successive heating cycle due to the bread cavity transmitting a certain amount of heat to the food item once the bread cavity has been warmed during previous heating cycles. In other words, the already warm bread cavity transmits a certain amount of heat to the food item in addition to the heat generated by the appliance during the heating cycle, resulting in less time being required to toast the food item. As a result, if the duration of the heating cycle is constant, food items placed in the bread cavity during subsequent heating cycles will be toasted more than those in previous heating cycles due to the additional heat transmitted from the bread cavity. To compensate for heat transmitted from the bread cavity to the food item during the second and subsequent heating cycles, conventional toasters include a timing circuit that compensates for this heat by reducing the duration of successive heating cycles.
FIG. 1 is a schematic of a conventional timer 10 that compensates for heating of the bread cavity by reducing the duration of successive heating cycles, as will now be described in more detail. In the timer 10, an external force is applied to close a switch 12 thereby applying an AC voltage from a voltage generator 14 to an input node 16 of the timer 10. An external lever (not shown) of the toaster containing the timer 10 is typically pushed down to apply the external force to close the switch 12 and to lower a food item into the bread cavity. The external lever is typically maintained in a down position by a mechanical latching mechanism (not shown) thereby maintaining the switch 12 closed. A coil 23 generates an electromagnetic force when energized to release the mechanical latching mechanism, thereby allowing the food item to be raised from the bread cavity and allowing the switch 12 to open, as indicated by the dotted line 25, as will be described in more detail below. However, when the switch 12 is closed, the AC voltage from the voltage generator 14 on the node 16 is rectified by a diode 18, and the magnitude of this rectified voltage is reduced by a voltage divider formed by series-connected resistors 20 and 22.
A capacitor 26 is coupled to a node 24 defined between the resistors 20 and 22, and filters the rectified voltage to provide approximately a DC supply voltage on the node 24. As explained below, a timing circuit 28 receives the supply voltage on node 24 and generates a first trigger signal V.sub.t1, on a node 29 a delayed time after the switch 12 is closed to apply the supply voltage to the circuit 28. The timing circuit 28 includes a resistor 30 and a variable resistor 32 connected in parallel with a resistor 34 and a thermistor 36. The thermistor 36 presents a resistance having a value that is a function of the temperature of the thermistor, as understood by those skilled in the art. The thermistor 36 has a negative temperature coefficient so that as the temperature of the thermistor increases, the value of the resistance presented by the thermistor decreases. Typically, the thermistor 36 is mounted near the bread cavity, of the toaster and thus presents a resistance having a value that is a function of the temperature within the bread cavity. The resistor 34 and thermistor 36 in parallel with the resistor 30 and the variable resistor 32 present an equivalent resistance R.sub.T between the node 24 and a capacitor 38 coupled between the node 29 and ground. The capacitor 38 and equivalent resistance R.sub.T together form an RC circuit with the voltage across the capacitor 38 having a value that varies as a function of time. The time dependence of the voltage across the capacitor 38 is determined by the values of the equivalent resistance R.sub.T presented by the resistors 30-34 and thermistor 36 and the capacitor 38, as well understood by those skilled in the art. In operation of the timing circuit 28, the voltage on the node 24 is applied through the equivalent resistance R.sub.T to charge the capacitor 38 and thereby develop first trigger signal V.sub.t1. The rate at which the capacitor 38 charges and thus the rate at which the magnitude of the first threshold signal V.sub.t1 increases is a function of the resistance presented by resistors 30-34 and thermistor 36, as previously described. A diode 52 and resistor 54 discharge the capacitor 38 when switch 12 is open.
A diac 40 receives the first trigger signal V.sub.t1 on a first terminal and has a second terminal coupled through series connected resistors 42 and 44 to ground. When the first trigger signal V.sub.t1 has a magnitude less than a predetermined breakdown voltage, the diac 40 presents a high impedance and no current flows through the diac. When the first trigger signal V.sub.t1 exceeds the breakdown voltage, the diac 40 turns ON and current flows from the node 29 through the diac 40 and series-connected resistors 42 and 44. The resistors 42 and 44 operate as a voltage divider, with the voltage across the resistor 44 being applied as a second trigger signal V.sub.t2 to a silicon controlled rectifier (SCR) 46, which is connected in series with the coil 23 and a resistor 50. When the second trigger signal V.sub.t2 exceeds a second breakdown voltage, the SCR 46 turns ON causing current to flow from the node 24 through the resistor 50 and coil 23, thereby energizing the coil. The resistor 50 reduces the magnitude of the voltage applied across the coil 23 when the SCR 46 is turned ON. As mentioned above, energizing the coil 23 releases a mechanical latching mechanism (not shown) to allow the switch 12 to open and the food article to be raised from the bread cavity.
The overall operation of the timer 10 during a heating cycle of a conventional appliance containing the timer will now be described in more detail. Initially, assume the switch 12 is open, isolating the voltage generator 14 from the node 16. To initiate a heating cycle, an external force is applied to close the switch 12 thereby applying the voltage from the generator 14 to the input node 16. When the voltage from the generator 14 is applied on the input node 16, the diode 18 rectifies this voltage and the supply voltage on node 24 is developed, as previously described. In response to the voltage on the node 24, the capacitor 38 begins charging at a rate determined by the value of the equivalent resistance R.sub.T presented by resistors 30-34 and thermistor 36. The variable resistor 32 is adjusted in relation to a "toast darkness" scale to control the duration of the heating cycle. As previously described, the thermistor 36 has a negative temperature coefficient so that as the temperature in the bread cavity increases the value of the resistance presented by the thermistor 36 decreases. Thus, as the temperature of the bread cavity increases, the equivalent resistance R.sub.T presented by the resistors 30-34 and the thermistor 36 decreases, causing the capacitor 38 to charge at a faster rate. The voltage across the capacitor 38 corresponds to the first trigger signal V.sub.t1, and as the capacitor 38 charges the magnitude of the first threshold voltage V.sub.t1 increases at a rate determined by the value of the equivalent resistance R.sub.T0. Once the first trigger signal V.sub.t1 reaches the breakdown voltage of the diac 40, the diac 40 turns ON causing current to flow through resistors 42 and 44. In response to this current flow through the resistor 44, the magnitude of the second trigger signal V.sub.t2 exceeds the breakdown voltage of the SCR 46, turning ON the SCR so that current flows through the SCR to thereby energize the coil 23. When the coil 23 is energized, the switch 12 opens, isolating the voltage generator 14 from the node 16 and thereby terminating the heating cycle of the appliance.
In a conventional toaster, when the coil 23 is energized causing the switch 12 to open a bread carriage within the toast cavity is typically released causing a portion of the toasted bread to extend beyond the top of the toaster so that it may be removed. It should also be noted that when the switch 12 opens causing the rectified voltage to be removed from the node 24, the capacitor 38 may discharge through the diode 52 and resistor 54 to thereby remove charge from the capacitor 38 so that residual charge remaining on the capacitor 38 does not adversely affect the time of subsequent heating cycles.
If the external force is again applied to close the switch 12 and initiate another heating cycle, the timer 10 operates in the same manner as previously described to energize the coil 23 a delay time after the switch 12 is closed. During this subsequent heating cycle, however, the bread cavity may still be warm from the previous cycle and thus the thermistor 36 presents a smaller resistance than during the prior heating cycle. As a result, the resistance R.sub.T presented by the resistors 30-34 and thermistor 36 is smaller than during the previous heating cycle, causing the capacitor 38 to charge more quickly and thereby reducing the delay time of the timer 10. More specifically, the signal V.sub.t1 more quickly exceeds the breakdown voltage of the diac 40, causing the diac to turn ON faster. As previously described, when the diac 40 turns ON, the signal V.sub.t2 is generated to trigger the SCR 46, energize to the coil 23, and terminate the heating cycle. Because the SCR 46 turns ON faster, the duration of the heating cycle is reduced accordingly. As previously described, this is desirable because toast placed in the bread cavity during the subsequent heating cycle will be toasted by a certain amount due to residual heat transmitted to the bread from the heated bread cavity. Thus, the delay time of the current heating cycle is decreased to toast the bread during the second heating cycle by the same amount as that during the first heating cycle.
Another conventional timer used in controlling the duration of heating cycles in a toaster includes a digital timer, such as an MC4541, coupled to a temperature sensitive capacitor. The capacitor functions as a temperature sensor, presenting a capacitance having a value that is a function of temperature. In operation, a coil is energized at the start of a heating cycle. The coil generates an electromagnetic force that is applied to hold the bread carriage within the cooking cavity during the heating cycle. During the heating cycle, the digital timer generates an oscillating signal having a frequency that is a function of the value of the capacitor. The frequency of the oscillating signal determines when the digital timer activates a transistor coupled to the coil to thereby de-energize the coil and terminate the heating cycle.
In the conventional timer 10, several factors make it difficult to maintain a consistent level of toasting during successive heating cycles. First, the precise location of the thermistor within the bread cavity is critical. The thermistor 36 must be positioned so that the resistance presented by the thermistor 36 varies as a function of the temperature in the bread cavity to properly adjust the delay time of the timer 10 and maintain consistent toasting among heating cycles. The position of the thermistor, however, may not be consistent from one toaster to the next, causing unwanted variations in the delay time of the timer 10. Another factor that adversely affects the levels of toasting is the inherent nonlinearity of the thermistor 36, which causes the delay time to be adjusted by amounts that do not properly compensate for increased temperatures in the bread cavity. The tolerance of the thermistor 36 is typically relatively large for less expensive thermistors, and such variations in the value of the resistance presented by the thermistor 36 among timing circuits 28 results in variations in the delay times among the timing circuits 28. An additional problem with the timer 10 may arise if the coil 23 fails "open." In this situation, when the SCR 46 turns ON, coil 23 is not energized so the switch 12 remains closed causing power to be continually applied to the toaster. This may result in a potentially dangerous situation as the toaster becomes increasingly hot. The prior art circuit including the digital timer and capacitor as described above does not present this same problem since the associated coil is energized at the start of a heating cycle and a failed open coil would prevent a heating cycle from being initiated.
There is a need for a timer to reliably control and adjust the duration of heating cycles in a toaster in order to maintain consistent levels of toasting of food items among successive heating cycles.